Okay so ... where to begin ?

My mom has me babysitting my sister every single Wednesday + weekends . Every week , I’ve been babysitting about 2-3 days average a week (13+ hours a day) since my family member is out of town for about 2 1/2 months. My hours at work have been cut due to me asking for so many days off every week here and there. I drive 40 minutes to my moms and 40 back wasting a ton of gas. She doesn’t pay me and has never really offered. I do it out of the kindness of my heart because I love my sister but I don’t know how to tell my mom I can’t do this anymore. It’s put a strain on my finances and my relationship with my bf of 3 years (who is also having severe mental health issues lately ) I’ve tried to tell her before that I can’t but she guilt trips me about all the terrible things that could happen to my sister in daycares or with people watching her who aren’t family. I’m stuck with what to do! Advice??